Particulate, absorbent, polymeric compositions are capable of absorbing large quantities of liquids such as water and body exudates (e.g., urine) and are further capable of retaining such absorbed liquids under moderate pressures. The absorption characteristics of such polymeric compositions make them especially useful for incorporation into absorbent articles such as diapers. See, for example, U.S. Pat. No. 3,699,103 (Harper et al), issued Jun. 13, 1972, and U.S. Pat. No. 3,770,731 (Harmon), issued Jun. 20, 1972, that disclose the use of particulate, absorbent, polymeric compositions (often referred to as "hydrogels", "superabsorbents", or "hydrocolloid materials") in absorbent articles.
Conventional particulate, absorbent, polymeric compositions, however, have the limitation that the particles are not immobilized and are free to migrate during processing and/or use. Migration of the particles can lead to material handling losses during manufacturing as well as nonhomogeneous incorporation of the particles into structures in which the particles are being used. A more significant problem, though, occurs when these particulate materials migrate during or after swelling in use. Such mobility leads to high resistance to liquid flow through the material due to the lack of stable interparticle capillary or liquid transport channels. This phenomenon is one form of what is commonly referred to as "gel blocking."
One attempt to overcome the performance limitations associated with absorbent particle mobility during use in absorbent articles is incorporation of the particulate, absorbent, polymeric compositions into tissue laminates, i.e. layered absorbent structures. By encapsulating the particles between tissue layers, the overall particle mobility within an absorbent structure is diminished. However, upon liquid contact, the particles within the laminate are often free to move relative to each other resulting in the breakdown of any pre-existent interparticle capillary channels.
Another attempted solution is to immobilize the particulate, absorbent, polymeric compositions by the addition of large quantities of liquid polyhydroxy compounds that act as an adhesive to hold the particles together or to a substrate. See, for example, U.S. Pat. No. 4,410,571 (Korpman), issued Oct. 18, 1983. While this approach does limit migration before and, to some extent, during swelling, the particles eventually become detached from each other in the presence of excess liquid, resulting again in the breakdown of any pre-existing capillary channels between the particles.
Another attempted solution to overcome the problem of absorbent particle mobility is to produce a superabsorbent film by extrusion of a solution of a linear absorbent polymer and subsequently crosslinking it. See, for example, U.S. Pat. No. 4,861,539 (Allen et al), issued Aug. 29, 1989 (crosslinked with a polyhydroxy compound such as a glycol or glycerol); and U.S. Pat. No. 4,076,673 (Burkholder), issued Feb. 28, 1978 (crosslinked with polyamine-polyamide epichlorohydrin adducts such as Kymene.RTM.). While these superabsorbent films may absorb significant quantities of liquids, they have limited liquid transport properties because they are essentially nonporous, i.e. lack internal capillary channels. Indeed, due to the lack of internal capillary channels, these superabsorbent films are especially prone to gel blocking.
Moreover, the crosslinking reaction between the hydroxy groups of the glycerol and the carboxy groups of the polymers present in the absorbent particles is relatively slow. Indeed, the glycerol treated absorbent particles are typically cured at 200.degree. C. for 50 minutes. This provides relatively brittle sheets of bonded absorbent particles that are more difficult to handle, especially in making the ultimately desired absorbent structures. Accordingly, these brittle sheets need to be treated with a plasticizer, such as a mixture of water and glycerol, to make them relatively flexible and thus easier to handle in manufacturing absorbent structures.
U.S. Pat. No. 5,324,561 (Rezai et al), issued Jun. 23, 1994, discloses an improved porous aggregregate macrostructure where the absorbent particles are crosslinked with cationic amino-epichlorohydrin adducts, such as Kymene. The use of these cationic, preferably polymeric, amino-epichlorohydrin adducts as the crosslinking agent improves the cure rate and enhances the absorbent capacity of the particles by reducing or eliminating innerparticle crosslinking. In addition, the cationic functional (e.g., azetedinium) groups of these adducts are believed to react very rapidly with the carboxy functional groups of the polymer material comprising the absorbent particles, even at ambient room temperature, e.g., at 18.degree.-25.degree. C.).
Even crosslinking the absorbent particles with amino-epichlorohydrin adducts has been found not to solve all the problems of these porous aggregate macrostructures. One is the flexibility of the aggregate macrostructure, especially when its in the form of a sheet or strip. The absorbent particles that make up these aggregate macrostructures are inherently rigid and hard, thus causing inherent inflexibility. In addition, at low relative humidites (e.g., about 20%) typically encountered during winter months, these aggregate macrostructures tend to crack and break. These aggregates macrostructures also become more rigid when subjected to higher temperatures (i.e., at 50.sub.-- C.) that typically occur during transport and storage of absorbent articles, such as diapers, in which these macrostructures are normally used. Such rigidity is undesirable in terms of the degree of comfort imparted to the wearer of absorbent article in which these macrostructures are contained.
Accordingly, it would be desirable to be able to make absorbent aggregate macrostructures of bonded absorbent particles that: (1) are more flexible; (2) impart a high degree of comfort to the wearer of the article; (3) do not tend to break or crack at low relative humidities typically encountered during winter months; (4) remain flexible even when subjected to higher temperatures typically encountered during transport and storage of absorbent articles, such as diapers; and (5) remain highly hydrophilic throughout transport, storage and use.
Briefly stated, the present invention meets the needs identifed above by providing porous absorbent macrostructures that comprise flexible interparticle bonded aggregates which are surface crosslinked. The porous absorbent macrostructures of the invention contain an effective amount of latex which is coated on a portion of the particles comprising the bonded aggregates so as to impart increased flexibility to the macrostructure.
In accordance with one aspect of the invention, a porous, absorbent, macrostructures that comprise flexible interparticle bonded aggregates is provided. These aggregates comprise: (a) a multiplicity of interconnected crosslinked particles comprising substantially water-insoluble, absorbent, hydrogel-forming polymer material; and (b) an effective amount of a latex to coat a portion of the particles so as to impart increased flexibility to the interparticle bonded aggregate. The latex is capable of being sintered at a temperature of about 25.sub.13 C. or lower; it is at least somewhat hydrophilic when sintered; and it has a Tg of about 25.sub.13 C. or lower when sintered.
In a preferred aspect of the invention, the interparticle bonded aggregate has pores between adjacent particles and the pores are interconnected by intercommunicating channels such that the macrostructure is liquid permeable, the circumscribed dry volume of the macrostructure being greater than about 0.008 mm.sup.3. Another aspect of the invention envisions the preferred latex being an emulsified polymer produced from an olefinic monomer selected from the group consisting of C.sub.2 -C.sub.4 alkyl and hydroxy alkyl acrylates, C.sub.1 -C.sub.4 alkyl or hydroxy alkyl methacrylates and mixtures thereof.
In another aspect of the invention, a method for imparting improved flexibility to these porous absorbent macrostructures is provided. This method comprises the steps of: (a) treating the porous aggregate macrostructure with an effective amount of a latex to coat a portion of the particles of the porous aggregate macrostructure, wherein the latex is capable of being sintered at a temperature of about 25.sub.13 C. or lower, is at least somewhat hydrophilic when sintered, and has a Tg of about 25.sub.13 C. or lower when sintered; and (b) sintering the latex treated porous aggregate macrostructure at a temperature of about 25.sub.13 C. or lower to impart increased flexibility thereto.
The porous, absorbent macrostructures obtained are useful, alone, or in combination with other absorbent materials, in absorbent structures for various absorbent articles, including diapers, adult incontinence pads, sanitary napkins, and the like. These porous absorbent macrostructures are more flexible and have greater stability at high temperature (e.g., about 50.sub.-- C.) and low relative humidity (e.g., about 20% ). This is due to the inclusion of certain latexes that are believed to coat at least some of the particles of the macrostructure to provide an elastic shell. These latexes are also at least somewhat hydrophilic so as not to adversely affect the fluid handling properties of the macrostructure.